This invention relates generally to sterilization processes. More particularly, this invention relates to a method and apparatus for monitoring or determining the effectiveness of a serilization process on a product that is sterilized inside a hermetic container.
In general, the concept of sterilization involves the elimination or at least the minimization of unwanted biological organisms from a particular environment. Food and drug products in containers, for example, may be sterilized by subjecting the containers to a heat sterilization process. In heat sterilization it is the high temperature acting over a period of time that results in the death of the unwanted biological contamination. Measuring temperature alone will not indicate the killing power of the sterilization treatment on bacteria; it is necessary to integrate the time and temperature of treatment. In the canned food industry, the sterilization cycle usually involves the use of steam to heat properly filled, sealed containers. To be effective, the sterilization process must function adequately even in the slowest heating zone of the container.
A widely used approach to the monitoring of sterilization processes employs bacterial spores. When a predetermined number of spores are subjected to the sterilization agent, death of the spores will proceed in a predictable manner. When the sterilization process is completed, a certain percentage of the test spores will have been killed depending on the severity of the treatment, the remainder will still be alive. When the surviving number of test bacteria is subsequently determined (through a variety of conventional techniques), a reasonable measure of the severity of the particular sterilization process will be obtained. This method of monitoring sterilization processes of course assumes that the death rate of microorganisms within the container will be approximately the same as the death rate experienced by the test biological organisms.
In the prior art, a variety of bacterial sterility process monitoring systems are known. One general group of such systems specifically for use in hospital autoclaves employs a predetermined number of viable microorganisms deposited on a piece of filter paper. The paper strips in envelopes may be used alone, or in one system, an adjacent cell containing a food or life sustaining substance separated by one or more reinforcing membranes or walls all in a compartment together. In this latter system the apparatus is subjected to the sterility process to be investigated, and afterwards withdrawn and manually crushed so that the bacteria or other microorganisms therewithin are fed by the nutrient medium enclosed in a vial. Detector material provided may subsequently indicate the effectiveness of the sterilization cycle in response to microorganism growth therewithin. Examples of systems of the above types include U.S. Pats. No. 3,611,717, issued May 9, 1972, U.S. Pat. No. 3,440,144 issued Apr. 22, 1969, U.S. Pat. No. 3,346,464, issued Oct. 10, 1967, U.S. Pat. No. 3,239,429, issued on Mar. 8, 1966, and U.S. Pat. No. 3,854,384 issued on Sept. 30, 1958. Each of the preceding patents discloses a sterilization monitor apparatus which may be subjected to the sterilization process along with the material to be sterilized. However, none of these preceding references disclose a means for rigidly positioning the apparatus within the critical slow-heating zone within a container to be sterilized or for isolating the sterilization-sensitive material completely and hermetically from either the product being sterilized or the heating medium that is acting as the sterilization agent. An example of the sterilization indicator system which employs chemicals rather than microorganisms or other life forms is seen in U.S. Pat. No. 3,627,469 issued on Dec. 14, 1971. The latter reference discloses a plurality of sterilization indicators which function by means of a change in color, depending upon the effectiveness of the sterilization cycle.
One of the biggest problems experienced with the use of microorganism or bacterial sterility indicators has been the ambiguity of the results achieved therefrom. Some prior art devices indicate only that bacteria have or have not survived the sterilization process. Moreover, it is difficult to ascertain what section of a container, for example, experienced the same heating effects as did the monitor cells employed. As mentioned, some of these prior art monitor cells are adapted to be manually squeezed after the cycle to free the nutrient medium so it can mix with the surviving microorganisms. While this factor may be desirable in certain circumstances, in a mechanized sterilization process where a vast quantity of cans or bottles, for example, must be sterilized, such systems are simply too structurally unsound to provide adequate or dependable results. Most prior art monitors are designed so that the heating medium comes in contact with the heat-sensitive agent during the sterilization cycle. Contact between the heating medium and the heatsensitive agent may be acceptable when steam is the heating medium; however, contact between any other heating medium or product and the sensitive agent may change the calibration of the sensitive agent and give erroneous results. Another problem with prior art sterilization monitors is that wires or leads are often required. This is true in the case of conventional thermocouples. When such lead wires are required, the object being tested often cannot be subjected to the sterilization process that is actually used in practice because of the complex path that the container must follow in proceeding through the sterilization equipment.